Cities and Nutrient Pollution

A photograph I took during my visit to the Hogsmill Sewage Treatment Works last week

My visit to the Hogsmill Sewage Treatment works last week got me thinking, and not just about the smell. I thought about Nitrogen and Phosphorus and questioned how cities factor into the biochemical cycles that I learned about in my lectures this week. What is the impact on the drinking water and the ecology of urban water bodies? So, stepping away from urban heat islands, I am going to turn my focus onto cities and nutrient pollution.

The bigger picture 

Human disruption of the Nitrogen and Phosphorus cycles has spurred a doubling and tripling of terrestrial flows of biological Nitrogen and Phosphorus respectively. Such nutrient pollution in cities is predominantly attributable to leaky sewage systems, with a contribution to riverine nitrogen input of 12% in the US, 25% in Western Europe, 33% in China and 68% in the Republic of North Korea. Differences in sewage system implementation explain this variations(see figure 1), with less than 35% of cities in developing countries possessing any form of sewage processing. The significance of cities for nutrient pollution is clear, challenging a preoccupation with agricultural activities as standalone leaders in biochemical cycle disruption.

Figure 1: Percentage of Sewage Treatment by Region: Source: (Selman and Greenhalgh, 2009)

Urban agricultural activities, particularly fertilizer use and industrial pollution have spurred eutrophication of urban water bodies, as seen at Murchison Bay of Lake Victoria(see figure 2). Intensification of land use to meet the demands of the skyrocketing population of nearby Kampala, with polluting runoffs of nitrogen and phosphorus rich surface water have resulted in a transformation of this water body. Accompanying untreated sewage flowing into Lake Victoria exposes populations to parasitic helminths, compromising public health.

Figure 2: Eutrophication and pollution in Lake Victoria, the result of intensified urban agricultural activities, untreated sewage discharge and intensified industrial practices in the surrounding regions, especially Kampala: Credit

Don't drink the water!

Beyond eutrophic and parasitic loads, in cities like Jakarta, nitrate pollution is of a greater concern for drinking water. Nitrate in drinking water above 50mg/l can lead to health complications, particularly methaglobinemia in infants. The Ciliwing river corridor that passes through Jakarta has contributed significant nitrate pollution to the groundwater which infiltrates the shallow aquifer, with 95% of leaking septic tanks as the main contributors. While within international limits,  gradually rising nitrate concentrations and population require policy reforms to counteract future hazards, in a city where the majority of water is accessed by privately owned pumping wells. While beautification projects have started to tackle the rubbish littering the Ciliwung, much remains to be desired for sewage treatment in Jakarta. This video gives more information:

Murky waters

Coastal dead zones are yet another major concern, particularly if these zones lay near cities. Masan Bay, encircled by the Korean cities of Masan, Changwon and Jinhee, is fed by over 1000 industrial sites and continuous domestic sewage inputs, red tides and algal blooms has earn't the body of water the title of most eutrophic bay in Korea. With the introduction of the total pollution load management system(TPLMS), chemical oxygen demand and land based nitrogen and phosphorus loads fell significantly from 2005-2010. However, total nitrogen and phosphorus in the bay remains stubborn to confront. The central government hopes to make the waters 'swimmable' by 2020, by upgrading existent waste water treatment plants to the Anaerobic/Anoxic/Oxic(A2O) system(see figure 4), which has been reported to remove up to 63% and 71% of Nitrogen and Phosphorus respectively. This is a technology with arguable potential for application in other regions with similar issues of inadequate sewage treatment.

Figure 4: A schematic of an A2O process modified with fiber polypropylene media. This form of biological nutrient removal(BNR) is popular in developing countries such as China and Vietnam. The A2O systems have proven highly effective in removing nitrogen and phosphorus from waste water processing - diagram source (Lai et al, 2011) 

Geo-engineering out the issue?

An interesting geo-engineering project carried out in the eutrophic Dutch urban ponds of Dongen and Eindhoven demonstrates alternatives for freshwater treatment. The application of a lanthanum-modified bentonite(LMB)(see figure 5) clay layer that captures phosphorus release from the sediment and bio-manipulation via introduction of macrophytes and control of carp populations saw a transformation of the lakes from eutrophic to clear, with significant phytoplankton biomass reductions. Needless to say, geo-engineering is currently  pretty controversial, but it is interesting to consider how the attitudes towards this practice may change in an urban context. I will be devoting my next posts to this, so stay tuned. This is really only a glimpse of issues and projects hoping to resolve current urban nutrient pollution, but fascinating nonetheless.

Figure 5: Schematic interpretation of the immobilization processes of Phosphorus in sediments amended Lanthanum modified bentonite(LMB).
Ions of bentonite in the clay are exchanged for lanthanum(La), which binds soluble reactive phosphorus(SRP) to form La-P, which is of extremely low solubility.
Thus, SRP concentrations are reduced in bodies of water where the LMB technique is applied.
Diagram Source: (Wang et al, 2017)